1. Field of the Invention
The present invention relates to a semiconductor laser device preferably applied to an optical disk system or the like and a method of manufacturing the same.
2. Description of the Prior Art
The Internet and electronic mails have rapidly come into wide use in recent years, leading to increasing enlargement of the personal computer market. Optical disk systems employing disk-shaped optical recording media such as CD-ROMs or DVD-ROMs are indispensable as the storage media for personal computers. In such optical disk systems, transition from a reproduction only type to a writing type and to a rewritable type is getting obvious.
A semiconductor laser device, which is a key device for an optical disk system, is strongly required to have a high output in order to improve the writing speed of the optical disk system.
FIG. 17 shows the basic structure of a typical conventional ridge type semiconductor laser device. In the case of a GaAs-based semiconductor laser device, for example, an n-type buffer layer 702 of n-type GaAs, an n-type cladding layer 703 of n-type AlGaAs, an emission layer of AlGaAs and a p-type cladding layer 705 of p-type AlGaAs are formed on an n-type semiconductor substrate 701 of n-type GaAs.
The p-type cladding layer 705 has a striped ridge portion provided with a central portion having a larger thickness than those of flat side portions for transverse mode control of the semiconductor laser device. An n-type blocking layer 706 of n-type AlGaAs is formed on the side surfaces and flat surfaces of the p-type cladding layer 705, in order to limit a current injection region.
Further, a p-type contact layer 707 of p-type GaAs is formed on the p-type cladding layer 705 and the n-type blocking layer 706. An n-type electrode 708 is formed on the rear surface of the n-type semiconductor substrate 701, while a p-type electrode 709 is formed on the p-type contact layer 707.
The p-type contact layer 707, having a smaller band gap than that of the emission layer 704, absorbs some of light generated in the emission layer 704. A laser beam is strongly confined in the stack direction of the semiconductor layers due to this absorption, to increase light density on the front facet of the laser device.
When the light density is increased on the front facet of the laser device, the aforementioned conventional semiconductor laser device is readily broken on the front facet. In order to increase the output of the semiconductor laser device, therefore, the light density on the front facet may be reduced by increasing the height H as well as the lower width W of the ridge portion.
When the height H of the ridge portion is increased in the semiconductor laser device shown in FIG. 17 provided with the ridge portion of a forward mesa structure having an upwardly reduced width, however, the width of the upper surface of the ridge portion is so reduced that a current hardly flows. Therefore, increase of the height H of the ridge portion is limited.
When the lower width W of the ridge portion is increased, on the other hand, it is difficult to confine light in the parallel direction to the junction plane and a horizontal divergence angle of the laser beam is abruptly reduced. Therefore, the difference between the horizontal and vertical divergence angles of the laser beam is increased to result in a problem such as deterioration of light gathering power or the like. When changing the width W, further, the horizontal divergence angle so remarkably changes that it is difficult to adjust the horizontal divergence angle.
In a semiconductor laser device provided with a ridge portion of a reverse mesa structure having an upwardly increased width, the width of the upper surface of the ridge portion is not reduced also when the height of the ridge portion is increased, and hence the aforementioned problem of the current is not caused in this case. However, a problem caused by increasing the bottom width of the ridge portion still remains.